1. Field of the Invention
This invention relates to a frequency conversion circuit for converting a frequency of a first frequency signal using a second frequency input signal and to a mixing circuit including the frequency conversion circuit.
2. Description of the Prior Art
A frequency conversion circuit having a single mixer comprising a dual gate FET (field effect transistor) for converting a frequency of a first input signal using a second input signal is known.
FIG. 18 is a schematic circuit diagram of a prior art frequency conversion circuit using a dual gate FET, In FIG. 18, numeral 1 is an input terminal for receiving a local oscillation (LO) signal. Numeral 2 is an input terminal for receiving a radio frequency (RF) signal. Numeral 3 is an IF (intermediate frequency signal) output terminal. Numeral 4 is a first gate bias voltage terminal. Numeral 5 is a second gate bias voltage terminal, Numeral 6 is a voltage source terminal. Numeral 7 is a matching circuit for matching the local oscillation signal with this frequency conversion circuit, Numeral 8 is a matching circuit for matching the radio frequency with this frequency conversion circuit. Numeral 9 is an IF matching circuit for matching the IF output signal with an external circuit. Numeral 10 is a dual gate FET having a pair of gates with respect to a pair of drain and source electrodes.
This prior art frequency conversion circuit using the dual gate FET is supplied with a supply voltage through the voltage source terminal 6 and biased with gate bias voltages applied to the first and second gate bias voltage terminals 4 and 5. The local oscillation signal is inputted to the dual gate FET 10 through the terminal 1 via the matching circuit 7 for impedance matching. The radio frequency signal is inputted to the dual gate FET 10 through the terminal 2 via the matching circuit 8 for impedance matching. The dual gate FET 10 mixes the local oscillation signal with the radio frequency signal to produce the IF output signal. The IF output signal is outputted through the matching circuit 9 with impedance matching.
This prior art frequency conversion circuit using the dual gate FET 10 can mix the local oscillation signal with the radio frequency signal with a relatively high frequency separation characteristic between the input terminals 1 and 2 for the local oscillation and radio frequency signals at a relatively low frequency. Thus, this structure requires no filter for the separation between the terminals of the local oscillation and radio frequency signals.
However, in the above-mentioned structure, if the frequencies of the radio signal and the local oscillation signal are set to the higher values, the original separation characteristic between the first and second gates becomes insufficient. Therefore, there is a problem that leaks of the local oscillation signal to the antenna side and the radio frequency signal to the local oscillation side become considerable, so that filters may be required to suppress the leaks.
Moreover, a mixing circuit including such prior art frequency conversion circuit for mixing the radio frequency signal and local frequency signal is known wherein matching circuits for impedance matching for amplifiers for radio frequency signal and local frequency signal are used. However, if the frequencies of the radio signal and the local oscillation signal are set to the higher values, the original separation characteristic between the first and second gates becomes insufficient. Therefore, there is a problem that leaks of the local oscillation signal to the antenna side and the radio frequency signal to the local oscillation side become considerable and matching circuit for impedance matching for amplifiers cannot prevent leak components from the intermediate frequency output from entering the frequency conversion circuit.